1. Field of the Invention
This invention relates to electronic music synthesis and in particular is concerned with generating waveshapes having points which are derivatives or integrals of prespecified musical waveshapes.
2. Description of the Prior Art
Musical tone synthesizers of the type in which a waveshape rich in harmonics is passed through a sliding formant filter to produce a time-varying tonal effect by altering the strength of the harmonics are well known in the musical tone generation art. In analog signal tone synthesizers, the sliding formant filter is generally implemented as a high pass or a low pass filter in which the cutoff frequency may be varied with time to alter the harmonic content of the musical waveshape at the output of the filter. In digital tone generation systems the equivalent effect of a sliding formant filter can be achieved by scaling the harmonic coefficients that are used in a computational algorithm to obtain the successive amplitude point values of the musical waveshape. Digital musical systems of this type are described in U.S. Pat. No. 3,956,960 entitled "Formant Filtering In A Computor Organ" and in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer."
In U.S. Pat. No. 4 281,574 an arrangement is described for a digital musical tone generation system which achieves a time-varying harmonic structure for a digital musical waveshape. Each amplitude point of the waveshape is modified by adding or subtracting a prior amplitude point. The delay time between the points can be varied so that any one of a number of prior data points can be selected for addition or subtraction with the current amplitude point. A source of sequential data representing changes in amplitude of the musical waveshape as a function of time are connected to a varialbe delay means, such as a shift register, which provides temporary storage of a group of sequential data points on a first-in, first-out basis. Any one of the data points stored in the shift register can be selected as one input to a subtract or add circuit together with the current data point from the source. Each resultant value is then applied to a digital-to-analog converter to provide an audio output signal.
A system related to the U.S. Pat. No. 4,281,574 is described in the technical article:
Kevin Karplus and Alex Strong, "Digital Synthesis of Plucked-String and Drum Timbres." Computer Music Journal, published by the MIT Press, Cambridge Mass., Vol. 7, No. 2, Summer 1983, pp 43-69.
In this article a basic tone modification system is described in which a new waveshape point is obtained by averaging two successive waveshape sample points. The difference of two successive points is also proposed as a method of changing the spectral content of a digital waveshape.
Both the system described in the referenced U.S. Pat. No. 4,281,574 and the technical article are related to a procedure of waveshape modification somewhat resembling successive integration or differentiation of a musical wavehshape.